Metal oxide semiconductor field effect (MOSFET) transistors or MOS transistors may have parameters greatly varied between different wafers and different batches of wafers. To reduce difficulties in designing a circuit and to allow a desired performance of the device, manufacturing processes need to be strictly controlled to thus control the process parameters to vary within a certain range. In the meantime, the wafers that are out of the performance range are scrapped to ensure the rest of the devices can satisfy the performance requirements.
Conventionally, a process sensor is used to detect the process corner of a MOS transistor in order to detect the effect of a current process on the circuit and to adjust the circuit in real time according to the process corner of the MOS transistor. The process sensor may be made based on a ring oscillator (RO), and the MOS transistor is used as an oscillation unit of the RO. The process corner of the MOS transistor is determined by detecting the oscillation signal of the RO output. In particular, each of the oscillation unit may include a PMOS transistor and an NMOS transistor, forming an inverter structure.
As shown in FIG. 1, the process sensor often detects three process corners of a MOS transistor, fast, slow and standard, to evaluate the overall performance of the MOS transistor. Specifically, the overall performances of the PMOS transistor and the NMOS transistor in each of the oscillation units are detected, and the detection result may include fast NMOS fast PMOS (corresponding to “FF” in FIG. 1), slow NMOS slow PMOS (corresponding to “SS” in FIG. 1), and typical NMOS typical PMOS (corresponding to “TT” in FIG. 1), which is sufficient for digital integrated circuits. However, for analog integrated circuits, the process corner detection results provided by this process sensor are non-sufficient, and the analog integrated circuits need to evaluate the performance of each of the internal NMOS transistors and PMOS transistors.